The present invention relates to an optical recording medium having a layer composed of a phase-changing material (referred to as a phase-changing material layer) served as a recording medium, and more particularly to the optical recording medium which enables to correspond to various line speeds.
In recent days, as a recordable optical disk, a phase-changing type optical disk has been proposed which contains a phase-changing material layer served as a recording one. As an example, this phase-changing type optical disk is formed to have a phase-changing material layer, a protective layer, and a reflective layer formed on a transparent substrate.
In this type of optical disk, the phase-changing material layer presents a crystalline state in its initial state. By applying writing light onto the phase-changing material layer, a minute area of the layer is changed into a liquid phase area. The movement of the writing light results in quenching the liquid phase area and thereby making the area non-crystalline, where pits are formed. The pits formed by non-crystallizing the area provide a lower reflective factor than the crystalline area. By detecting the difference of the reflective factor, it is reproduced as an information signal.
As this type of a recording system to an optical disk, a pit position recording system or a mark length recording system has been proposed. The pit position recording system is arranged to form a recording pit at a corresponding spot to data 1 of an original signal. Generally, this system is used because the formed pits are relatively stable.
On the other hand, the bit length recording system is arranged to form a recording pit in a manner to make an inverted spot of the data of the original signal, that is, the spot wherein data 0 is changed into data 1 or data 1 is changed into data 0 coincide with the edge position. This pit length recording system is more beneficial in light of density than the pit position recording system though the tail end of the pit may be different from the front end of the pit in some recording materials. From this beneficial point, this system is likely to use the phase-changing type optical disk.
By the way, various kinds of line speeds of the writing light given when writing the pits on the optical disk are set according to their way of uses. For example, for a compact disk, a line speed of 1.2 to 1.4 m/sec is set. For a digital video disk, a line speed of 4.0 m/sec is set. For an optical disk used for a camera, a relatively fast line speed of 6.0 to 14.0 m/sec is set. For these disks, the writing light is scanned at a constant line speed, while for the optical disks used for computers, the writing speed is scanned at a constant angular speed. In this case, the line speed of the writing light is progressively changed from the outer peripheral side to the inner peripheral side inside of the disk plane. The line speed of the outer peripheral side is 2.5 times as fast as that of the inner peripheral side.
In the phase-changing material layer, however, the quenching speed given after liquid-phasing has influence on the process of changing the phase. For example, if the line speed of the writing light is set to a slower one, the quenching speed is made slower, so that the formation of pits is made unstable.
That is, for forming non-crystalline minute areas (pits) on the phase-changing material layer, after the phase is changed into the liquid phase by applying the writing light, it is necessary to quickly cool down the liquid-phased area.
However, when the moving speed of the writing light is made slower, the heat is left in the liquid phased area. Hence, the liquid-phased area is not quenched after moving the writing light. In actual, the area is gradually cooled down. As a result, the area to be non-crystallized is re-crystallized, so that a predetermined pit form cannot be obtained. In particular, the pit length recording system is likely to have a longer pit length than the pit position recording system, so that the laser beam may be continuously applied onto the phase-changing material layer. In such a case, the heat is likely to be left in the phase-changing material layer, so that the layer is likely to be re-crystallized.
For overcoming this disadvantage, a trial has been made for adding an additive to the phase-changing material layer for suppressing the re-crystallization after the area is liquid-phased. If the additive is used, the addition amount is important. For example, if the amount of the additive is too great, it is difficult to crystallize the non-crystallized area when erasing the pits, unfavorably, the pits to be erased are left. Hence, it is necessary to use only the minimum necessary amount of the additive for suppressing the re-crystallization. In actual, however, the proper amount of additive is not sufficiently studied.